The present invention relates to optical disks formed with grooves and lands for optically tracking an optical signal. More particularly, the present invention relates to optical disks having data which may be stored, played back, or erased from both lands and grooves.
Optical disks have a capacity for high density data storage and high speed data playback. In this regard, optical disks are particularly suited for consumer applications requiring the use of a computer. Conventional storage disks such as disks having a diameter of 5.25 inches or 3.5 inches, as well as magneto-optical disks or phase change disks capable of data re-writing, have been standardized by ISO standards.
A conventional magneto-optical disk, as illustrated in FIG. 6 (PRIOR ART), has a spiral formed from alternating concave and convex patterns extending outward from an inner circumference of the disk. The spiral is a guide for tracking data on a disk substrate 1. The data is tracked by a laser spot from a pickup 2 of a recording and playback device which follows the spiral.
The spiral is a tracking guide which has been defined in an ISO standard. In the ISO standard, concave portions 3 which are farthest away from pickup 2, i.e. remote portions, are called "lands." On the other hand, convex portions 4 which are closer to pickup 2 are called "grooves." The distance from the center of a land 3 to the center of an adjacent land 3 is called "track pitch," (P).
Generally, the depth (d3) of the grooves 4 is 50 nm, the width (W) is 0.4-0.6 .mu.m, and the track pitch (P) is 1.4 .mu.m. However, recent improvements in the art of optical disks have a narrowed track pitch with a track pitch (P) of 1.1 .mu.m. This facilitates higher density data recording.
To enhance the recording density of such optical disks, the diameter of a beam spot which illuminates the surface of the optical disk may be made smaller. Methods to form a smaller beam spot having a reduced diameter include a method of shortening the wavelength of the light beam and a method of enlarging the numerical aperture (NA) of an objective lens.
However, the method of shortening the wavelength of the light beam is limited by the wavelengths of the semiconductor lasers used as a light source of the light beam. Moreover, the problem is that the form and power of short wavelength lasers are insufficient. On the other hand, the method of enlarging the numerical aperture (NA) of the objective lens of the recording and playback device is difficult because a high degree of technology is necessary to control aberration characteristics in the objective lens.
Consequently, there has recently been proposed a technique called "magnetically induced super resolution" ("MSR" hereinbelow), to increase apparent resolving power of a magneto-optical disk when playing back data which has been recorded at high density. The MSR technique increases apparent resolving power even when the wavelength of the light source and the size of the light spot used for playback are left unchanged from their conventional values.
The MSR method is a method of playing back recordings of data points which are smaller than the beam diameter. The MSR method reduces an effective playback aperture by magnetically forming an optical mask. The mask utilizes a rise in temperature distribution of an optical disk medium within the light spot due to the light itself and rotary motion of the optical disk medium. Thus, a portion of the signal which enters the light spot is masked from being detected as a playback signal.
In the MSR method, there is a Front Aperture Detection ("FAD") mode which masks a high temperature portion, a Rear Aperture Detection ("RAD") mode which masks a low temperature portion, and various other modes in which a magnet is necessary for operation. By way of the MSR method, playback resolving power is increased above the limitations of the optical system and recording density is increased in a linear density direction.
Recently, as another method of narrowing track pitch, techniques have been proposed for recording data on both lands and grooves (hereinbelow "land/groove recording methods"). The land/groove recording methods are in contrast to conventional recording methods which record data on either one of lands or grooves. The conventional recording methods utilize either a land or a groove as a guide between tracks. However, the land/groove recording methods virtually reduce the track pitch to half when compared with conventional recording methods. This effectively doubles the data storage capacity.
Nevertheless, in the land/groove recording methods, because track pitch (P) becomes half of the track pitch of the conventional storage medium, a number of problems develop. A significant problem is thermal crosstalk at the time of recording. Other problems include signal crosstalk between adjoining lands and grooves at the time of playback. A magneto-optical disk or a phase change disk is a thermal recording disk which requires a rise in temperature of the disk substrate due to interaction with the laser light. Thus, if the gap of adjacent tracks becomes small, thermal diffusion to the adjacent tracks becomes large. When first writing data to a track and then erasing data from the track, data written on adjacent tracks becomes erased. This is termed "thermal crosstalk" or "cross-erasure."
In a land/groove recording method, because both lands and grooves are used as recording tracks, both types of tracks are easily affected by cross-erasure. Moreover, during repeated recording, control of cross-erasure becomes increasingly important. For example, in conventional optical disks, the width of lands and grooves becomes limited to about 0.9 .mu.m by problems of cross-erasure. Accordingly, track pitch can not be increasingly narrowed, which thereby becomes an obstacle to high density recording.